The short title is "Content-ID-in-SIP", whereas it seems more natural
to use "Content-ID in SIP".

Table of Contents

     1.1.  Setting up ID value uniquely identifying body . . . . . .   2
     1.2.  Referencing the ID value uniquely identifying body  . . .   3

This seems an awkward way of phrasing it.  In particular, there seems
to be no reason to emphasize uniqueness here.  Perhaps

     1.1.  Establishing a Content-ID identifying the body
     1.2.  Referencing the Content-ID value identifying the body

--

     5.1.  Header Field  . . . . . . . . . . . . . . . . . . . . . .   6
   6.  Change Log  . . . . . . . . . . . . . . . . . . . . . . . . .   7
   7.  Normative References  . . . . . . . . . . . . . . . . . . . .   7
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   8

These items capitalize later words, whereas the other items capitalize
only initial words.  It seems that the current RFC style heavily
favors capitalizing all the "important" words in section titles.

1.  Introduction

   A message-body of the SIP message can contain one body only or can
   contain several body parts, as specified in [RFC3261] and [RFC5621].

Strictly speaking, a SIP message containing several body parts also
has a body (whose MIME type is multipart/something).  I think it would
be better to phrase this

   A message-body of the SIP message can be a non-multipart body or
   can be a multipart body that contains several body parts, as
   specified in [RFC3261] and [RFC5621].

I think this can be carried through to other places that use the
phrase "one body".  (Although "one body" is a close version of the
phrase "unitary body", which would be a good term for "non-multipart
body", though nobody uses the phrase!)

1.5.  Solution

Section 1.5 looks much like section 3.3, and I think the duplication
should be reduced.  Perhaps the text in 1.5 could be reduced to

   The Content-ID header field included in header fields of a SIP
   message identifies a body part consisting of the message-body of
   the SIP message.  The metadata provided by some additional header
   fields also applies to the message body as a whole.

3.2.  Syntax

   The ABNF for the Content-ID header fields is:

   Content-ID = "Content-ID" HCOLON msg-id

   NOTE: msg-id is specified in [RFC5322].

Comparing with RFC 5322, I see:

   msg-id          =   [CFWS] "<" id-left "@" id-right ">" [CFWS]

   CFWS            =   (1*([FWS] comment) [FWS]) / FWS

   comment         =   "(" *([FWS] ccontent) [FWS] ")"

I'm sure we don't want to allow comments!

I can see a couple of ways to avoid this.  One is to annotate it:

   The ABNF for the Content-ID header fields is:

   Content-ID = "Content-ID" HCOLON msg-id

   NOTE: msg-id is specified in [RFC5322], but may not contain
   "comment".

But this allows RFC 5322's FWS, which is slightly broader than RFC
3261's SWS.  Probably best is to force the whitespace to be in the
style of RFC 3261:

   The ABNF for the Content-ID header fields is:

   Content-ID = "Content-ID" HCOLON msg-id

   msg-id     = "<" id-left "@" id-right ">"

   NOTE: id-left and id-right are specified in [RFC5322].

Note that SIP header fields generally do not allow trailing
whitespace.

3.3.  Semantic

I think you want "Semantics" as the section title.

I notice this item in the list (which is duplicated in section 1.5):

   o  a Content-ID header field, if included in the header fields of the
      SIP message;

This reads awkwardly, because the beginning of the paragraph has
specified that there is a Content-ID header field in the message.  I
think this could be clarified by promoting this item to the top of the
list and explicitly labeling it all as metadata:

   The Content-ID header field included in header fields of a SIP
   message identifies a body part consisting of the message-body of
   the SIP message and the following metadata:

   o  the Content-ID header field itself;

and then continuing with "a MIME-Version header field", etc.

Dale